The invention relates to the making and breaking of tubing strings, such as drill strings and casing strings, suspended in a well drilled or being drilled in the lithosphere while continuous circulation of a fluid such as mud or foam is maintained.
An apparatus for connecting and disconnecting tubulars and a tubing string suspended in a well and for axially displacing that tubing string, and methods of assembling and disassembling a tubing string projecting into a well are known from U.S. Pat. No. 3,559,739. In this document a method and an apparatus for providing continuous foam circulation in wells is disclosed. To enable the circulation to continue while a tubular is connected to the tubing string or disconnected from the tubing string, a shell is provided enclosing a space where the connection is to be made or broken. When the upper end of the tubing string is opened, the flow via the top drive is taken over by a flow which enters or leaves the tubing string via the back-up conduit and the connecting chamber formed by the shell in which the connection is being made up or broken out. To allow the entry of a tubular to be added into the connecting chamber or the removal of a tubular from the connecting chamber, while maintaining the flow through the open end of the tubing string, the connecting chamber can be divided in two portions by a preventer. Thus, the flow can be maintained via the high pressure back-up conduit and the lower portion of the connecting chamber while a tubular is brought into a position in-line with the tubing string or is transported away from above the tubing string.
Other examples of such a method and such an apparatus are disclosed in International Patent Application WO 98/16716, which corresponds to U.S. Pat. No. 6,315,051, which issued to Ayling on Nov. 13, 2001 and which is incorporated herein by reference.
A problem of these methods and apparatuses is to accurately control the axial movement of and the axial forces exerted by a tubular to be connected to or being disconnected from the tubing string. Especially the stabbing forces immediately before making up a coupling and the retraction forces during the completion of breaking of a coupling are difficult to control. Inaccuracies in the control of stabbing forces can easily lead to damage to the coupling members, for instance to the threads. Apart from the costs of repair or replacement, this also entails the risk of coupling failure after the coupling has entered the well, with the associated hazards and extra operating costs.
It is an object of the present invention to provide an apparatus and a method with which control over stabbing and retraction forces is less impeded by the pressure in the connecting chamber and which allows to build up and remove the pressure in the connecting chamber quickly without loosing control over stabbing and retraction forces.
According to one aspect of the present invention, this object is achieved by providing an apparatus of the above-identified type having a pressure compensating structure for compensating axial force exerted by pressure in an upper portion of a connecting chamber pressing a tubular projecting from the upper portion of the connecting chamber in an axial direction out of the connecting chamber. The pressure compensating structure includes at least one pressure chamber and at least one pressure transfer member formed by the connector or by a separate pressure transfer member connectable to a tubular. The pressure chamber has a passage and means for sealing off the passage against the at least one pressure transfer member, the at least one pressure chamber is connected to the connecting chamber for maintaining a pressure in the at least one pressure chamber corresponding to pressure in at least the upper portion of the connecting chamber. The at least one pressure transfer member is displaceable in the at least one pressure chamber and is arranged for transferring a force axially, by pressing the tubular into the connecting chamber in reaction to pressure in the at least one pressure chamber. According to other aspects of the present invention, this object is achieved by, after lowering the tubular into the upper one of the passages, maintaining a pressure in at least one pressure chamber at a level corresponding to pressure in the upper portion of the connecting chamber, axially urging the tubular towards the connecting chamber and at least partially balancing out upward pressure exerted to the tubular by the pressure applied to the upper portion of the connecting chamber. Alternatively, according to other aspects of the present invention, this object is achieved by, after lowering the at least one connector into the connecting shell, maintaining pressure in at least one pressure chamber at a level corresponding to pressure in the upper portion of the connecting chamber, axially urging the connector towards the connecting chamber and at least partially balancing out upward pressure exerted to the connector by the pressure applied to the upper portion of the connecting chamber.
It has been found that the stabbing and retraction forces are difficult to control because the pressure in the upper portion of the chamber in the connecting shellxe2x80x94which can be in the order of magnitude of 500 barxe2x80x94pushes the tubular to be connected or being disconnected out of the connecting chamber with a force which is substantially larger than the stabbing and retraction forces and which varies with fluctuations of the pressure in the chamber of the connecting shell which occur in operation.
The present invention provides that the tubular is pushed towards the connecting chamber by the pressure in the pressure chamber with a force which is at least essentially proportional with the force with which the tubular or the connector is pushed out of the connecting chamber by the pressure in the upper portion of the connecting chamber. Thus, the other axial forces which have to be transferred to the connector or the tubular to control the retraction or stabbing forces are substantially reduced, substantially more constant and require no or substantially less compensation for the forces generated by pressure in the connecting chamber. The devices for controlling and generating the stabbing and retraction forces and movements are substantially relieved from having to compensate the axial forces generated by the pressure in the chamber in the connecting shell. Therefore, the retraction and stabbing forces are better controllable and/or can be controlled with less powerful drives. Moreover, the pressure in the connecting chamber can be built up and removed quickly and even during stabbing or completion of breaking a coupling, without substantially disturbing the control of the stabbing or retraction forces and displacements of the tubular.
Particularly advantageous modes of carrying out the invention are set forth in the dependent claims.
Further details, objects, features and advantages of the invention are described with reference to the drawings.